The present invention relates generally to stitching two or more images. More specifically, the present invention provides methods, systems, and software for modifying a signal intensity of one or more images before or after they are stitched.
In the medical imaging field, oftentimes the field of view of the imaging devices is smaller than the anatomy being examined. Consequently, two or more individual radiographic projection images (hereinafter referred to as “projection images” or “images”) need to be obtained and then properly assembled to form the appropriate field of view for analysis. Such assembly of the images is referred to hereinafter as “stitching.”
The need for stitching is encountered in many digital radiography, MRI, ultrasound, and nuclear medicine evaluations, all techniques that are capable of imaging along an axis of possible motion. Unfortunately, stitching of the images is not always straightforward. Because it is not always known how much the patient or the imaging device has moved or how much the patient shifts or rotates between image shots, accurate stitching of the individual images often proves difficult. Thus, flexibility of the stitching the images is desirable.
A stitched image is made up of two or more images, arranged so as to match anatomic features of interest. FIG. 1A illustrates one imaging device 12 that comprises an x-ray source and a plurality of overlapping image plates that are used to obtain a plurality of projection images that are stitched together to form a full image of the subject tissue (e.g., leg/spine image). In general, the intensity scale is assumed to be the same in all the individual projection images that are used in the stitched image. For example, U.S. Pat. Nos. 6,273,606, 6,195,471, 6,097,418 and 5,986,279 relate to stitching images, but none of these patents deal with signal intensity differences between the plurality of projection images.
There are two commonly used intensity scales in image display. One intensity scale sets a Window and a Level (W and L). For example, in an intensity scale of 0-32,000, the features of interest may only occupy the signal intensity region between 5,000 and 7,000. Typically, it is desirable to use the full display scale (which may be only 256 or 512 levels of gray) to show the brightness of only the range of interest. In this case, the operator will set L=6,000 and W=2,000, so that the center of the window is at 6,000 and the range is ±1,000, i.e., from 5,000 to 7,000. By this scheme, an intensity of 5,000 is mapped to zero (or minimum brightness) and an intensity of 7,000 is mapped to maximum brightness for that particular projection image.
Another intensity scale includes an upper and lower level (U and L). As the name implies, L selects the image intensity value that will mapped as 0 intensity in the display and U selects the image intensity value that will be mapped as maximum in the display. In the example above, for this intensity scale, L would equal 5,000 and U would equal 7,000 for this intensity scale.
As can be appreciated, there exist other intensity scales, some non-linear, some reversing scales, and the present invention is not limited to the particular intensity scale used to map image intensity. Changing the bounds of the intensity scales is referred to herein as “windowing” or changing a “windowing scale.”
There are various reasons why the individual images can have non-matching intensity scales, e.g., why the same anatomic landmark in an overlap area may be more intense in one image than the other. For example, if the x-ray images are acquired by separate exposures, there can be differences in the x-ray tube settings and/or performance. Even if the plurality of images are acquired in a single exposure, however, each detecting plate of the x-ray imaging device may have a somewhat different sensitivity, or the settings of the digitizer can vary. Such intensity variations are referred to herein as “uncontrolled variations.”
There is another reason why the same anatomic landmark in an overlap area may be more intense in one image than the other. For example, consider the best case scenario, in which a single exposure with perfectly matched plates and a stable digitizer is used, in which there are just two plates. One of the plates is over the chest and the other plate is over the abdomen. The spine runs along both the chest and abdomen body sections and plates. For simplicity, even if it is assumed that all the vertebral bodies in the spine have the same x-ray absorption, over the length of the spine in the abdomen there are solid organs, muscle and fat, and the total absorption of the beam will be high. If the image is a negative, the spine is very bright and may be the brightest pixel of the image. In contrast, over the chest, there is less solid tissue over the spine, and the absorption is consequently less. Thus, the chest image would consequently be less bright than the abdomen image. In such a scenario, the brightest pixel of the chest image may be less bright than the brightest pixel of the abdomen image.
In addition to the uncontrolled variations and the variations in the images caused by subject tissue itself, because of limited dynamic range, manufacturers of the digitizers used in the x-ray imaging devices may choose to apportion or “squeeze” the whole available intensity scale between the maximum and minimum brightness pixels. Therefore, the absorption value of a particular pixel in an overlap section of the stitched image could be assigned to different image intensity values, depending on the intensity level of the maximum and minimum pixel in each of the images that are stitched. For instance, the brightest pixel in each the chest and in the abdomen image, although one less bright than the other, would be assigned the maximum value (e.g., an image value of 32,000). This is typically done by applying a simple scaling factor. This process is called a “scale factor variation.”
Once the stitched image is created, the operator may still need to set different windowing for different parts of the body, as the spine may appear saturated in the abdomen when the windowing is appropriate for the chest. Given that the radiologist starts with different images, it may be desirable to window each individual image separately.
It is to be noted that the setting of intensity values in the present application differs from parent application (Ser. No. 10/005,473). In the parent application, pixel intensity blending was concerned only with the overlap section of the stitched image. It was generally assumed that each individual image had the “correct” exposure, and that these exposures were the same for all the images being stitched. In the parent case, the blending methods used in the overlap section were intended to provide a smooth transition, with a minimum of artifacts, and to provide a pleasing perception of the overall image.
Accordingly, what are needed are methods, software, and systems that provide an accurate means for stitching images. It would be desirable to provide a highly versatile set of choices that can increase the ease of stitching. It would further be desirable to provide improved quality of the stitched image, especially in controlling the signal intensity of one or more of the images used in the stitched image.